Created
May 4, 2014 21:02
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(* test data *) | |
let answer = Div (Div (Con 1972, Con 2), Con 23);; (* 42 *) | |
let error = Div(Con 1, Con 0);; (* Exception: Division_by_zero. *) | |
(* term type *) | |
type term = Con of int | Div of term * term ;; | |
(* primitive evaluator *) | |
let rec eval = function Con a -> a | |
| Div (t, u) -> eval(t) / eval(u) ;; | |
(* with exception handling *) | |
type eval_exception = string;; | |
type 'a m = Raise of eval_exception | Return of 'a;; | |
let rec eval x = match x with | |
| Con a -> Return a | |
| Div (t, u) -> match eval(t) with | |
| Raise e -> Raise e | |
| Return b -> match eval(u) with | |
| Raise e -> Raise e | |
| Return c -> if c = 0 then Raise ("divided by my zero") | |
else Return ( b / c) ;; | |
(* with state *) | |
(* without explicitly type declearation *) | |
let rec eval term x = match (term, x) with | |
| ((Con a), x) -> (a, x) | |
| (Div (t, u), x) -> let (b,y) = (eval t x) in | |
let (c,z) = (eval u y) in | |
(b / c, z + 1) ;; | |
type state = int ;; | |
let rec eval (exp:term) (x:state) : int * state = match (exp, x) with | |
| ((Con a), x) -> (a, x) | |
| (Div (t, u), x) -> let (b,y) = (eval t x) in | |
let (c,z) = (eval u y) in | |
(b / c, z + 1) ;; | |
(* which one is correct in above three declearration? *) | |
type 'a m = 'a * state;; | |
type 'a m = state -> 'a * state;; | |
type 'a m = 'a -> 'a * state;; | |
type ('a, state) m = 'a * state;; | |
(* only this one is right *) | |
type m 'a = state -> 'a * state;; | |
let rec eval term : int m = match term with | |
| (Con a) -> fun x -> (a, x) | |
| (Div (t, u)) -> fun x -> | |
let (b,y) = eval(t)(x) in | |
let (c,z) = eval(u)(y) in | |
(b / c, z + 1);; | |
(* output *) | |
( * somethig is wrong with this) | |
(* helper function for pretty print *) | |
let rec showterm = function Con a -> "Con " ^ (string_of_int a) | |
| Div (x, y) -> "Div (" ^ showterm(x) ^ "," ^ showterm(y) ^ " )" ;; | |
let line t a = (showterm t) ^ " = " ^ (string_of_int a) ^ " => " ;; | |
let rec eval term = match term with | |
| (Con a) -> (line (Con a) a, a) | |
| (Div (t, u)) -> let (x,a) = eval t in | |
let (y,b) = eval u in | |
(x ^ y ^ (line (Div (t, u)) (a/b)), a/b);; | |
(* with type * *) | |
type output_step = string ;; | |
type m 'a = output_step * 'a ;; | |
let rec eval term : int m = match term with | |
| (Con a) -> (line (Con a) a, a) | |
| (Div (t, u)) -> let (x,a) = eval t in | |
let (y,b) = eval u in | |
(x ^ y ^ (line (Div (t, u)) (a/b)), a/b);; | |
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